Semiconductor devices such as power semiconductor devices include an edge termination structure for high voltage relief at the lateral periphery of the semiconductor device. Minimizing the space needed by edge termination while maintaining the blocking capabilities of the device is an important consideration. In addition, edge termination structures should minimize the influence of surface charges and external electrical fields.
Commonly known edge termination structures employ different measures such as field rings, so-called laterally varying doping, trench structures, and edge structures of the wafer with positive or negative grinding angle. A measure providing a high ruggedness against surface charges and external electrical fields employs so-called field-plates that can also be combined with other measures. The field-plates cause a deformation and spreading of the electrical field lines in the region of curved pn-junctions to reduce the electrical field. However, due to the manufacturing processes used, field-plates have sharp edges that cause field peaks in the semiconductor material and in the dielectric layer arranged between the field-plate and the semiconductor material.
Field peaks caused by the sharp edges of the field-plates can in principle be avoided by optimizing the shape of the field-plates. However, such optimized field-plate geometry is difficult to manufacture. Therefore, field-plates with multiple steps are commonly used that partially reduce the field strength but do not completely avoid the field peaks in the dielectric layer underneath the field-plates. As a result, long term problems still remain. Furthermore, multiple photolithographic steps are needed for manufacturing field-plates with multiple steps.
Another commonly used approach is etching abrupt or steep edges by anisotropic etching. This approach is limited to the formation of flank angles larger than 10°. Furthermore, only thin oxide layers can be processed so that the range of use of this approach is limited.
In view of the above, there is a need for improvement.